Activate the Inspect button (found in issues listed on the Current Document tab only).
The Inspect button opens the HTML tab in Firebug and:
Something useful to note about Firebug's HTML tab: When you hover over a piece of code, the corresponding item is highlighted in the webpage view.
If you are viewing issues on the Report tab, switch to the issue listing on the Current Document tab:
Go back to: Working with Issues - Overview.